There is a critical gap in our knowledge regarding the molecular mechanisms by which the intestinal microbiota influence epithelial cell cycle regulation and stem cell dynamics during the initiation and progression of GI cancers. This gap represents a barrier to scientific progress because, until it is addressed, an explanation for conditions resulting from dysbiosis between the gut microbiota and the host will continue to be beyond our understanding. Our long-term goal is to identify the cellular signaling pathways, the bacterial community structure, and the microbial products that mediate the influences of the microbiota on human health. The objective of this proposal is to identify how perturbations to the microbiota influence intestinal stem cell (ISC) turnover, and by extension tumor initiation or progression -and ultimately, how deliberate manipulation of the microbiota may offer a therapeutic strategy. Based on our preliminary data, our central hypothesis is that specific members of the highly host adapted microbiota (particularly lactobacilli) have co-evolved to facilitate intestinal cell proliferation by inducing the generation of ROS which then regulate cell signaling in the gut epithelium. Given that a subset of the microbiota possess potent pro-proliferative potential, we further hypothesize that altered, absolute or relative numbers of the microbes will have consequent effects on epithelia growth dynamics, and particularly in cases of intestinal injury, may play a role in intestinal tumor initiation and progression. The rationale fo this hypothesis is the well-established notion that physiological generation of low levels of ROS by the action of host NADPH oxidases in distinct subcellular domains act as critical second messengers in multiple signaling networks. In addition, our published and preliminary data identify well- characterized oncogenic cell signaling pathways that are modulated by bacterial-induced ROS generation. Furthermore, it is well-established that physiological generation of low levels of ROS in distinct subcellular domains act as critical second messengers in multiple signaling networks due to their ability to reversibly oxidize low pKa cysteines (sulfur switches) of specific sensor target proteins. Based on these compelling preliminary data generated by our research group, the central hypothesis will be tested in three specific aims; 1) Characterize the signaling pathways that mediate microbiota-induced stem cell proliferation, 2) Identify the influence of manipulated microbiota in model epithelial early neoplasia, and 3) Identify symbiotic bacteria and bacterial communities that induce redox dependent cell signaling. Our approach will employ, ex-vivo enteroid model, knockout mice, a novel redox I-CAT proteomic technique, and a highly innovative genetically tractable Drosophila model whose biology can be manipulated to a far greater extent than mammalian models. The outcomes of these investigations will have an important positive impact on public health because of direct implications to idiopathic intestinal cancers. The investigation is also relevant to the mission of NIDDK/NIH by addressing preventative interventions for these conditions.